The ability of the dark-adapted visual system to count absorbed photons has been known for many years; however, the biophysical mechanisms which make photon detection and counting possible are not understood. Much of rod vision occurs at light levels where photon absorptions occur rarely; thus, failure of either transduction of single photons or transmission of the resulting signals to bipolar cells severely impairs rod vision. Visual sensitivity places strict constraints on how absorbed photons are transduced into electrical signals and how these electrical signals are transmitted across the rod-bipolar synapse. Current understanding of signal transduction and synaptic transmission cannot account for this level of performance. Experiments proposed here will determine how accurately information about the number and timing of photon absorptions is represented in the rod signals, what source of noise limits this accuracy and what mechanisms permit reliable transmission of these responses to bipolar cells. Studies of phototransduction will investigate the functional significance of the rod's ability to produce a nearly identical response to each absorbed photon. In particular, we will test the idea that this reproducibility is critical for the temporal sensitivity of rod vision - that is the ability to represent the time of photon absorption accurately. These studies will also characterize the other noise sources in the rod signals and determine their contribution to limiting the detection and temporal sensitivity of the rod's signals. These are requisite measurements to further understanding of the relationship between photoreceptor signal and noise and visual sensitivity. Studies of synaptic transmission will determine how single photon responses are reliably transmitted to bipolar cells in the retina. Two issues will receive particular attention: (1) separation of single photon response from other noise sources in the rods by a thresholding nonlinearity in signal transfer; and (2) changes in the kinetics of the dim flash response in signal transfer from rods to bipolars. Signal transfer will be studied directly by controlling the rod voltage while recording the resulting bipolar response. These experiments will further the general understanding of how synapses in non-spiking cells work and the relation between synaptic mechanisms and visual sensitivity.